This invention relates in general to electrostatography and, in particular, to the preparation of electrophotographic imaging members having multiple layers. Flexible electrostatographic belt imaging members are well known in the art. Typical electrostatographic flexible belt imaging members include, for example, photoreceptors or photosensitive imaging members for electrophotographic imaging systems, and electroreceptors or ionographic imaging members for electrographic imaging systems. These belts are usually formed by cutting a rectangular sheet out from a web, overlapping the opposite ends of the cut sheet, and ultrasonically welding the overlapped ends together to form a welded seam belt.
In electrophotography, a flexible electrophotographic imaging belt or the like (imaging member) containing a photoconductive insulating layer on a conductive layer is imaged by first uniformly electrostatically charging its surface. The imaging member is then exposed to a pattern of activating electromagnetic radiation such as light. The radiation selectively dissipates the charge in the illuminated areas of the photoconductive insulating layer while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the surface of the photoconductive insulating layer. The resulting visible image may then be transferred from the imaging member directly or indirectly to a support such as paper. This imaging process may be repeated many times with reusable imaging members.
An electrophotographic imaging member may be provided in a number of forms. For example, the imaging member may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite imaging member comprises a layer of finely divided particles of a photoconductive compound dispersed in an electrically insulating organic resin binder. Layered photoreceptor of a typical electrophotographic imaging member having separate photogenerating and charge transport layers is disclosed in U.S. Pat. No. 4,265,990. The photogenerating layer is capable of photogenerating charge and injecting the photogenerated charge into the charge transport layer.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements, including narrow operating limits, on photoreceptors.
The numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One type of multi-layered photoreceptor that has been employed as a flexible belt in electrophotographic imaging systems comprises a substrate, a conductive layer, a blocking layer, an adhesive layer, a charge generating layer, and a charge transport layer. This photoreceptor may also comprise additional layers such as an anti-curl backing layer and an overcoating layer.
When one or more photoconductive layers are applied to a flexible supporting substrate the resulting photoconductive imaging member tends to curl. Curling is undesirable for a number of reasons. During the electrophotographic imaging process, curling may result in non-uniform distances from a charging device. Non-uniform distances produce non-uniform charging, resulting in variations in high background deposits during development of the electrostatic latent image. Further, a curled imaging member requires considerable tension to flatten against a supporting member. Where the support comprises a large flat area for full frame flash exposure, the imaging member may tear while flattening. Further, belts from flattened, curled members are more likely to incur stress induced cracks during cycling. These cracks print out on the final electrophotographic copy. Member belts subjected to high tension to remove curling also are more vulnerable to develop premature mechanical failure due to belt creep and dynamic fatigue if used in belt module designs utilizing small roller sizes (e.g. 19 mm or smaller).
An anti-curl coating may be applied to the side of the supporting substrate opposite the photoconductive layer to counteract the imaging member curling. However, application of such a coating requires an additional operational step and additional materials thereby reducing production through-put and increasing cost. Further, anti-curl coatings have not been wholly satisfactory. The anti-curl coating may wear off after extended belt machine functions. The anti-curl coating may also delaminate and separate during functioning under service conditions of high temperature and high humidity rendering the photoconductive imaging member unacceptable for forming quality images. A photoconductive imaging member requiring an anti-curl coating to render imaging member flatness has added overall cross-sectional thickness which increases the bending stress of the imaging member causing it to reduce the resistance of the imaging member surface to fatigue stress during cyclic function over the machine belt support rollers. Fatigue stress leads to the development of cracks in the charge transport layer as well as seam delamination. In an imaging member belt, the presence of the anti-curl coating at the overlapped ends of a cut piece imaging member of a seam joint also increases the volume of molten mass ejected out in the ultrasonic seam welding process to form a larger seam splashing. The seam splashing interacts with the cleaning blade which has been found to cause blade wear problems in electrophotographic imaging and cleaning processes.
Moreover, an anti-curl coating reduces transparency. Hence, an anti-curl coating reduces the performance of electrophotographic imaging systems requiring rear exposure of electromagnetic radiation to activate imaging member back erase during the electrophotographic imaging process. Although the reduction in transparency may in some cases be compensated by increasing the intensity of the electromagnetic radiation, such increase is generally undesirable due to the amount of heat generated as well as the greater costs necessary to achieve the higher intensity.
U.S. Pat. No. 4,983,481 relates to an imaging member that addresses curl problems and discloses a method to effect the elimination of the need for an anti-curl coating by providing a substrate layer with a linear thermal contraction coefficient substantially identical to the linear thermal contraction coefficient of the charge transport layer. Many of the substrate materials disclosed in U.S. Pat. No. 4,983,481 provide satisfactory linear thermal contraction coefficients for matching the linear thermal contraction coefficient of a particular applied charge transport layer. However, these materials are otherwise unsatisfactory because they fail during the imaging member manufacturing process or in service under machine operating conditions. For example, substrate materials may develop cracks during solution coating of other layers in the imaging member manufacturing process.
Further, some imaging members characterized by substrates having a linear thermal contraction coefficient substantially identical to the linear thermal contraction coefficient of the charge transport layer fail under conditions of applied belt tension at elevated temperatures in an imaging machine environment. In manufacturing, a web of imaging member is subjected to a tension during the production coating process and to an elevated temperature of about 115.degree. C. During machine operation, the belt is constantly subjected to a tension and to a machine cavity temperature of about 50.degree. C. Under these conditions, some imaging members having substrates of matched linear thermal contraction coefficient may exhibit poor resistance to heat or low yield point which develops into permanent dimensional deformation problems. These substrate materials, for example, include polyvinyl fluoride resins such as TEDLAR.TM. and amorphous polyterephthalate resins such as MELINAR.TM.. Thus, the curl-free characteristics of photosensitive imaging members comprising known substrate materials of matched linear thermal contraction coefficient have been unsatisfactory.
Conventional photosensitive members, photosensitive member preparation methods, and calendering processes are disclosed in Nagashima et al., U.S. Pat. No. 5,547,704; Leseman et al., U.S. Pat. No. 5,173,141; Yu et al., U.S. Pat. No. 5,518,854; Yu et al., U.S. Pat. No. 5,415,961; and Yu et al., U.S. Pat. No. 5,606,396.